Great to hear you like Agenda so much, and you’ll like even bettie that collapsing notes is already possible by:
On Mac:
- double-clicking the title area of the note
- using the menu options or keyboard shortcuts in the Note menu
On iOS
- double-tapping the title area of the note
- using the menu option after clicking the cog wheel in the bottom right corner of a selected note
- using the pinch-gesture on a note